The human spine consists of individual vertebras that are connected to each other. Under normal circumstances, the structures that make up the spine function to protect the neural structures and to allow us to stand erect, bear axial loads, and be flexible for bending and rotation. However, disorders of the spine occur when one or more of these spine structures are abnormal. In these pathologic circumstances, surgery may be tried to restore the spine to normal, achieve stability, protect the neural structures, or to relieve the patient of discomfort. The goal of spine surgery for a multitude of spinal disorders, especially those causing compression of the neural structures, is often decompression of the neural elements and/or fusion of adjacent vertebral segments. Fusion works well because it stops pain due to movement at the facet joints or intervertebral discs, holds the spine in place after correcting deformity, and prevents instability and or deformity of the spine after surgical procedures such as discectomies, laminectomies, or corpectomies. Discectomy and fusion or corpectomy and fusion are most commonly performed on the cervical spine but there is also increasing application on the thoracic and lumbar spine.
Several spinal fixation systems exist for stabilizing the spine so that bony fusion is achieved. The majority of these fixation systems utilize fixation elements such as rods, wires, or plates that attach to screws threaded into the vertebral bodies, facets, or the pedicles. Because the outer surface of the vertebral body is typically non-planar and the structure of the vertebras is relatively complex, it is important that the fixation elements (e.g., rods, plates, wires, staples and/or screws) are properly aligned when they are inserted into the vertebras. Improper alignment may result in improper or unstable placement of the fixation element and/or disengagement of the fixation element. Achieving and maintaining accurate positioning and guidance of these fixation elements, however have proven to be quite difficult in practice. Such positioning difficulties are further complicated by the fact that the alignment angle for a fixation device through one vertebral body or pair of vertebral bodies will be unique to that individual due to individual differences in the spinal curvature and anatomies. Accordingly, there is a need for a spinal fixation assembly that provides flexibility and polyaxial positioning of the fixation elements, preferably independent from each other.